Process and device for non-destructive determination of the prestressing condition of ferromagnetic securing elements

ABSTRACT

The process described enables in situ testing of securing elements, for example bolted assemblies, to verify that their tightness is adequate and to detect any material defects. To this end, it is sufficient that a coil (19) subjects at least one part of the screwed assembly to an alternating magnetic field. This field, whose frequency can be varied, provides a spectrum containing information about the tightness of the assembly.

BACKGROUND OF THE INVENTION Field of the Invention

The invention relates to a process for the non-destructive determinationof the stressing condition of ferromagnetic securing elements, in whichprocess an alternating magnetic field is applied to said elements andthe magneto-mechanical acoustic emission thereof is detected andevaluated.

Determining the stressing condition which occurs in a securing elementat its site of use is a topical problem of vast significance. Itssignificance resides above all in the aspect of safety. However, theeconomic aspect of this problem is not to be underestimated, either. Inthis context, it may also be pointed out that all over the world,product liability has been playing an ever-increasing part.

What is especially significant is the determination of the stressingconditions prevailing in screwed connections. Whenever parts are to bedetachably connected, this is as a rule done by means of screwedconnections. In a motorvehicle, for example, about two to three thousandscrewed connections can be found. Of these, about three hundred arecrucial for safety. For this reason, these screws or bolts have to betightened particularly carefully, i.e. nether too loosely nor toostrongly (cf. "Industrieanzeiger" 13/1988, pp. 24 through 27).

When a screw or bolt is tightened, the ultimately decisive factor is thetension force. It is the tension force alone which decides on howtightly the parts are to be detachably connected. At present, the manskilled in the art does not disposed of any direct methods for measuringthis force; he can merely conclude from indices or auxiliary magnitudes,namely the torque or the pivoting angle or a combination of the two.Furthermore, the yield point may also be considered to be an auxiliarymagnitude in a certain sense.

When a screw or bolt is tightened under torque control, it must be takeninto account that sixty to ninety per cent of the torque are required asfriction under the screw or bolt head and in the thread. It is thereforethe remaining ten to forty per cent of the torque alone which effect astrain on the screw or bolt. This strain on the screw or bolt alonecauses the desired tension force. Under different friction conditionsand with constant tightening moment, the tension force can vary up to ahundred per cent in the extreme case. For verifying the tension force ofa screwed connection, detecting the torque thus remains merely amakeshift measuring solution. Deviations from the required tension forcein screwed connections can be reduced by the use of a tightening processwhich is controlled by both the torque and the pivoting angle. In thisprocess, however, the expenditure for electronic controlling andmeasuring equipment is far higher than with torque-controlledtightening. In a tightening process which is controlled by thetorque/pivoting angle, the screw or bolt is turned in until a plasticdeformation occurs, from which point onwards the screw or bolt istightened by a predetermined angle. Since the strain of the screw orbolt can be determined from the pivoting angle and the pitch of thethread, the tension force of the screw or bolt may be calculated, takinginto account the elastic modulus of the screw/bolt material.Irrespective of the friction conditions, this yield point-controlledtightening process provides the best indirect indications about thepre-stressing force of a screwed connection.

As a rule, it would be ideal for determining the prestressing force ifthe parameters stress and strain could be measured directly during thetightening of the screw or bolt. In production, however, whenconventional tightening processes were used, it has so far beenimpossible to measure these parameters ("Industrieanzeiger" 16/1988, pp21 through 24).

German patent application DE-AS 25 19 430 already discloses a measuringprocess for determining the axial force of screw bolts in which thescrew bolts are excited to ultra-sonic resonance vibrations by means oftransversal and longitudinal waves. The natural frequencies are thusdependent on the longitudinal modulus and the transversal modulus, resp.of the screw bolts. The axial forces to be measured are derived fromshifts of the natural frequencies as a result of the strain of thesample body.

Moreover, German patent DE 33 33 285 A1 teaches means for detectingforces, stresses and accelerations in machines, devices and the like, inwhich a measuring value generator is built into a structural elementunder stress. The measuring value generator contains a low-Ohmresistance structural element. In screwed connections, the measuringvalue generator is built into the bolt or the washer.

Further, German patent DE 33 45 760 C2 discloses measuring meanscomprising a measuring spring made of ferromagnetic material, in whichthe deformation behaviour of an elastic spring of ferromagnetic materialis examined by means of a strain measuring strip and a magnetic fieldwhich passes through the spring at the site of the strain measuringstrip.

German patent DE 27 14 334 A1 teaches means for monitoring the tensionforce of a screwed connection, in which a tightly fitting measuring wireis wound around the periphery of the nut of a screwed connection formeasuring the stress-related radial expansion. One end of the measuringwire is secured to the nut, the other end remaining tangentially movableon its periphery. After the prestressing force has been exerted on thenut, the latter expands radially, resulting in the measuring wire beingdisplaced. This displacement is detected longitudinally and assigned tothe prestressing force.

Furthermore, German utility model DE-GM 6608345 discloses a detectingelement in the form of a washer which transforms the appliedprestressing force into an electrical signal.

For detecting stresses in materials having ferromagnetic properties,magnetic measuring processes may be used. These involve, for example,recording the Barkhausen effect, measuring the permeability or multipledetecting of magnetic parameters. The physical causes of the stresssusceptibility of ferromagnetic materials are based on magnetoelasticeffects which, after material deformation or interior rearrangementprocesses, result in changes in the energy contributions determining thedomain structure. These changes then influence the magnetic parameters.

German patent DE 34 04 232 relates to a process for examining materialproperties and material states of ferromagnetic materials on the basisof the magneto-mechanical acoustic emission of the ferromagneticmaterial in a magnetic field. In this process, the magneto-elasticallyinduced emission of a standardized ferromagnetic sample and of theferromagnetic material to be examined is recorded at a continuouslyvaried frequency of the magnetic field and at a constant amplitude ofthe magnetising field strength of the alternating field, after which theacoustic emissions of the standardized ferromagnetic sample and theemissions emanating from the ferromagnetic material under test are thencompared. In this process, a modifiable portion of a direct-currentfield is also intended to be superimposed on the alternating magneticfield. This modifiable portion of the direct-current field is used toadjust a certain point of the hysteresis curve for premagnetizing thesample or for demagnetising a previously magnetized sample.

It may further be gathered from a report of a meeting (W. Stengel:"Magnetoelastische Resonanzspektroskopie". Deutsche Gesellschaft furzerstorungsfreie Prufung e.V. (German Society for non-destructiveTesting), Berichtsband (Volume) 14, 1988, pp 638 through 645) that asample which is subjected to a tensile force is subsequently found tohave a characteristic magnetostrictive vibration behaviour in every areaof the corresponding stress-strain diagram. Plastic deformation of thesample can be seen from a frequency shift of related resonanceamplitudes. In this case, the magnetostrictive vibration was inducedexclusively along the longitudinal measuring area of the tensile sampleby means of a cylindrical coil surrounding the longitudinal measuringarea. The acoustic emission behaviour thus can only be related to thetensile stress condition prevailing in this area.

SUMMARY OF THE INVENTION

Based on this prior art, it is the object of the invention to provide aprocess and a device by means of which the stress condition and thestrain of a ferromagnetic connecting element, e.g. a screwed connection,may be determined during or after tightening, regardless of the frictionconditions prevailing.

This object is accomplished according to the invention by a process forthe non-destructive determination of the stressing condition offerromagnetic connecting elements, in particular built-in connectingelements, in which an alternating magnetic field is applied to theconnecting element, the resulting magnetostrictive or acousticvibrations are detected and the signal obtained is evaluated.

The process characterized in that a variable D.C. magnetic field issuperimposed on the alternating magnetic field.

The process characterized in that the D.C. magnetic field is modulatedin a quasi static manner such that its variable strength, starting froman initial value, is varied cyclically between two field strengthvalues.

The process characterized in that the frequency of the alternatingmagnetic field is varied and that a magneto-mechanical acousticvibration spectrum is detected as a function of the frequency.

The process characterized in that the vibration spectrum obtained duringtesting the connecting element is compared with a vibration spectrumassigned to a stress-strain curve in order to obtain a measuring resultfor the stressing condition.

The process characterized in that reference values are determined from astandardized element, which reference values are then used for creatingreference values in the stress-strain curve, the vibration spectrumproduced during the test of the connecting element being related tothese reference magnitudes in order to obtain a measuring result for thestressing condition.

The process characterized in that a numerical value is determined whichis in a functional relation to the area under the determined vibrationspectrum and which is assigned a point on the stress-strain curve.

The process characterized in that a numerical value is determined whichis in functional relation with the area of the magnetostrictivehysteresis curve and which is assigned a point on the stress-straincurve.

The process characterized in that an output signal obtained afterevaluation is fed into display means for displaying the measured valueor is used for controlling screw means.

The process characterized in that the frequency shift of individuallines of a spectrum is measured in comparison with a reference spectrumand obtained as a measure for the strain of a connecting element.

The process characterized in that the amplitude change of individuallines of the spectrum is measured in comparison with a referencespectrum and obtained as a measure for the stress in the connectingelement.

The process characterized in that the stress-strain curve is obtained byconnecting the measured values of the frequency shift and the amplitudechange.

When a screwed connection is tightened, a multi-axial tension conditionbuilds up in the screw or bolt. A tensile stress is thereby exerted onthe area of the clamped length of the screw or bolt. The parts connectedand tightened by the screw or bolt are thus under compressive stress.The compression stresses in the screwed components are not limitedexclusively to the area directly under the screw/bolt head, but at L_(K)=8d even extend to up to D_(B) =3D_(k) along the clamped length L_(K),if the dimensions of the components permit this, as can be seen fromFIG. 2 and 3. D_(K) is the head or nut contact diameter. High notchstresses are to be expected in the transient range from shaft to headand in the area of the thread. In the screw/bolt head tensile andcompression stresses occur simultaneously. In nut and bolt connections,the nut is predominantly under compression stress (Decker, Karl-Heinz:"Maschinenelemente, Gestaltung und Berechnung", 8th ed., Edits.Hanser-Verlag, 1982, p. 142).

In ferromagnetic materials, the formation of the structure of themagnetic area (Weiss' domains) is influenced both by stresses in theinterior of the material and by mechanical stresses affecting thematerial from the outside. According to theoretical reflections (CharlesKittel: "Physical Theory of Ferromagnetic Domains", in "Reviews ofModern Physics", Vol. 21, NO. 4 (1949), pp 541 through 583) the array ofthe magnetic domains can be roughly described, taking four energycontributions into account. The structure is formed under the conditionthat the sum of the energies taking part therein becomes minimal. Themagnetostrictive behaviour of a ferromagnetic material, i.e. its changeof shape in the presence of a magnetic field, is thus substantiallydetermined by the magneto-elastic energy contribution which, forisotropic solids, depends on the magnetostriction constants and themagnitude of the mechanical stresses. The magnetostriction constantcharacterizes the average spontaneous grid distortion which occurs atthe same time as the spontaneous magnetisation within the magneticdomains (Reviews of Modern Physics, vol. 21, no. 4, October 1949, pp.541, 583).

Therefore, if a material having magnetic properties is exposed tomechanical stresses from the exterior, it is not only the material'smagnetostrictive behaviour which changes but also other stress-dependentmagnetic magnitudes, such as its permeability or the magnetic inductiondepending thereon. Thus, as shown in FIG. 1, materials having positivemagnetostriction generally show an increase of their permeability andinduction values when exposed to mechanical tensile stresses increasingto the elastic yield point of the material. On the other hand,permeability and consequently also induction decrease with stressesleading to strains above the elastic yield point, as set out in R. M.Bozorth: "Ferromagnetism", D. Van Nostrand Company, Inc., New York,1955, pp. 595-599.

It may thus be assumed that a screwed connection mounted in its finalposition will feature a magnetic domain structure in the areasinfluenced by the stresses, which structure is determined amongst othersby the prevailing stress condition. If thus an external magnetic fieldis locally directed to the stress-affected areas and themagnetostrictive behaviour is detected, then this measurement indirectlyalso detects the stresses prevailing at this site. This is the casebecause the magnetostrictive behaviour is directly dependent on theprevailing stresses.

The invention is based on the novel finding that the prestressingcondition characteristic of the clamping force of a connecting element,e.g. a screwed connection, can also be tested after it has beenassembled and installed at its site of use, provided that the externalmagnetic field only reaches (at least) a partial area of the connectionwhich is exposed to the stress condition created by the connection. In ascrewed connection, it is thus sufficient only to detect a partial areainterspersed with stress lines, even if the stress condition prevailingthere is different as compared to the stress condition prevailing in theshaft area of interest. The fact that the force flux or the course ofthe stress lines is concentrated in the area of the connecting elementand closed in itself, the stress conditions prevailing in differentpartial areas are in their effects directly related to those in theshaft area.

It has thus been found that e.g. in the case of a screwed connection, itis not absolutely necessary that also the shaft of the screw or bolt beaffected by the external magnetic field, but that it is sufficient if anaccessible part of the screwed connection, e.g. the screw/bolt head isacted upon by the magnetic field. The present invention thereforeprovides a novel process which makes it possible, even if the shaftitself is inaccessible, to determine the stress in the shaft which isimportant for the safety of the connection.

The invention also relates to a device for performing the process. Thedevice is characterized by the the non-destructive determination of astressing condition of ferromagnetic connecting elements, in particularof built-in connecting elements, with an A.C. generator (21) of variablefrequency, a coil (19) connected with the a.C. generator (21) forgenerating a magnetic field in at least part of the connecting element(13), an acoustic-electric sensor (23) abutable to the connectingelement (13) and signal processing and control means (25).

The device characterized by a D.C. generator (22) connected with thecoil (19) for generating a D.C. magnetic field.

The device characterized by a D.C. field coil (20) for generating amagnetic field in at least part of the connecting element (13), the D.C.field coil (20) being connected with a D.C. generator (22).

The device characterized in that display means (27) for displaying thestressing condition in the connecting element (13) is connected with theevaluating circuit (25).

The device characterized in that screw means (28) is connected with theevaluating circuit (25).

The device characterized in that the coil (19 or 20) and the sensor (23)are arranged practically concentrically to each other.

The device characterized in that the sensor (23) is mounted on acoupling element (26).

The device characterized in that the coil (19 or 20) is disposed withinthe coupling element (26).

The device characterized in that it is provided in the form of arod-like hand device having a head (33) in which the coils (19 or 20)and the sensor (23) are accommodated.

The device characterized in that the sensor (23) can be displayedaxially in the head (33) against the force of a spring (37).

The device characterized in that the alternating field coil (19) and theD.C. field coil (20) are essentially arranged concentrically to eachother.

The device characterized in that the D.C. field coil (20) is disposedessentially perpendicularly to the alternating field coil (19).

The device characterized in that the D.C. field coil (20) is disposed ona coupling element (26), which coupling element (26) on the one handtransfers the measuring signals from the excitation site to the sensor(23) and on the other feeds the D.C. field flux into the connectingelement (13) to be tested.

The device characterized in that the coils (19, 20) are integrated in anautomatic hand device.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is further explained with reference to the drawings.

FIG. 1 is a diagram indicating the course of the permeability as afunction of an external tensile stress

FIG. 2 shows the force flux in a screwed connection, at the same timeschematically illustrating the device for performing the process,

FIG. 3 shows the force flux in a screwed connection with a stud,

FIG. 4 is a schematic illustration of a coil and sensor for performingmeasurements in a hexagon socket screw,

FIG. 5 is a schematic illustration of coil and sensor during measurementof a hexagon socket screw,

FIG. 6 is a schematic illustration of an embodiment example in the formof a hand device for non-destructive measurement of the stress conditionin a connecting element,

FIG. 7a shows vibration spectrograms during measurement of a structuralcomponent before stressing,

FIG. 7b shows vibration spectrograms during measurement of a structuralcomponent after stressing thereof,

FIG. 8 (including parts 8a-8e) shows the areas A, B, C and D of astress-strain diagram,

FIG. 8b shows a stress-strain diagram corresponding to area A,

FIG. 8c shows a stress-strain diagram corresponding to area B,

FIG. 8d shows a stress-strain diagram corresponding to area C, and

FIG. 8e shows a stress-strain diagram corresponding to area D,

FIG. 9 indicates measuring results obtained during testing of a screwedconnection, in which the prestressing condition of the screw/bolt wasincreased by changing the tightening moment,

FIG. 10 illustrates measuring results of several corresponding screwedconnections as in FIG. 9, said screwed connections each having adifferent prestressing condition

DESCRIPTION OF THE INVENTION

FIG. 1 shows the above-mentioned influence of a mechanical tensilestress on the maximum magnetic permeability in a material havingmagnetostrictively positive properties.

FIG. 2 shows a screwed connection which connects parts 11 and 12 bymeans of bolt 13 and nut 15. Reference numeral 17 indicates the stresslines.

The schematically illustrated device for performing the processaccording to the invention comprises a coil 19 connected to an A.C.generator 21, which generator in turn is controlled by a signalprocessing and control means 25. The frequency of this A.C. generator 21can be varied in a range of approx. 0.1 kHz to up to about 1 MHz. Apiezo-electric sensor 23 is coupled to an accessible part of the bolt13. This sensor 23 is connected to a signal processing and control means25. The control means 25 supplies signals to display means 27 and/orscrew means 28. The measuring device illustrated can thus also be usedfor terminating the tightening procedure after a certain stresscondition has been reached.

The process according to the invention is suited for examining variouskinds of screwed connections, for instance also a connection with stud13', as is shown in FIG. 3. Basically, however, other kinds ofconnecting elements which are exposed to a stress at their site of usecan be tested in this manner.

FIG. 4 shows a screwed connection with a hexagonal socket screw 13".Hexagon socket screws of this kind are also referred to as Allen screws.Again, a coil 19 is shown which is capable of applying an alternatingmagnetic field to at least one voltage-carrying part of the hexagonsocket screw 13". The sensor 23 is in contact with the screw 13' via thecoupling element 26, which is accommodated in the hexagon socket opening27 of the hexagon socket screw head 29.

As is shown in FIG. 5, it is also possible to accommodate coil 19 in thehexagon socket opening 27. Again the sensor 23 is in mechanical contactwith the screw head 29 via a coupling element 26. There are variouspossible practical embodiments of the measuring device schematicallyillustrated above. FIG. 6 for example shows a rod-like hand device 30having a handle 31 and display means 27. The head 33 of the device 30holds the coil 19 and the sensor 23. Sensor 23 is mounted on a shaft 35which in rest position is held in the indicated position by a spring 37.If the head 33 is put over a screw or bolt head, a nut or a part of adifferent connecting element, then the sensor 23 rests on the connectingelement. Actuation of the press head 39 starts the measuring process,the result of which can then be read from display means 27. The currentsource, the frequency generator and the evaluation circuit, if madesmall enough, can be accommodated inside the hand device 30. Otherwise,they can be connected by means of a cable 41.

It is desirable, but not absolutely necessary, that the sensor 23 be incontact with the connecting element 13. Since sound waves propagate wellin metals, the sensor 23 could for example also be put onto theworkpiece 11. If desired, it is also possible not to mount the excitingcoil and/or the sensor in the manner of the illustrated examples, butfor instance rather dispose them laterally on the accessible part of theconnecting element. Deviating from the figures illustrated, coilconfigurations are thus possible in which the alternating magnetic fieldacts unsymmetrically with regard to the part of the connecting elementor the stress field.

The process and the device can also be used for testing connectingelements, in which the individual parts forming the connection do notexclusively have ferromagnetic properties. In the case of a screwedconnection for example, it will suffice if either the nut or the screwbolt is ferromagnetic and at the same time exposed to the stress fieldof the connection.

FIG. 7 (including parts 7a and 7b) shows the changes occurring in theamplitude spectrum of a sample if the latter is strained. Before e.g. ascrew or a bolt is tightened and forces as shown in FIG. 2 are at work,an amplitude spectrum is obtained, as is exemplarily shown in the toppicture of FIG. 7. If a stress is exerted which leads to a plasticdeformation, then a spectrum is obtained, as can be seen exemplarily inthe lower picture of FIG. 7. The spectrum obtained is comparativelylittle structured but still has sharply formed resonance points. Acomparison with the top picture shows that a frequency shift hasoccurred.

As a rule, it can thus be concluded that a frequency shift is indicativeof the strain and an amplitude change is indicative of the stressprevailing in the material of the individual part. The evaluation of thevarious spectra makes two things possible. On the one hand, astress-strain diagram can be drawn up for the material of the structuralcomponent. On the other, the stress condition of a structural componentcan be determined, i.e. at which point in the stress-strain diagram itis.

FIG. 8 (including parts 8a-8e) shows the interrelation of the changingspectrograms a, b, c, d with the stress-strain diagram. Immediatelyafter a tensile stress has occurred, the resonance behaviour changesmarkedly as compared to the initial spectrum as shown in the top part ofFIG. 7. What is particularly conspicuous here is that spectracorresponding to the linear-elastic range of the stress-strain diagramdiffer strongly from those corresponding to the plastic deformationrange of the stress-strain diagram. The transition from the elastic tothe plastic deformation range is marked by sharp and less structuredresonances. This change in the amplitude spectrum is already perceptiblein spectrum b.

FIG. 8 merely shows (as FIGS. 8b-8d, respectively) four spectra a, b, c,d, as are typical of the corresponding areas a, b, c and d of thestress-strain diagram. In reality, a different spectrum is obtained foreach point of the stress-strain curve. Thus, if the stress-straindiagram of the respective material of a component is known, then it maybe determined from the corresponding spectrogram which point in thestress strain curve was reached by tightening the connecting element.The point in the stress-strain curve can be determined for instance bycalculating the area under the spectrum curve and concluding therefromthe corresponding point in the stress-strain curve.

FIG. 9 shows measuring results obtained in testing a screwed connectionin which the prestressing of the screw or bolt was increased by varyingthe tightening moment. The magnetostrictive vibration for determiningthe prestressing condition is thereby induced by an A.C. field coilsurrounding the nut of the screwed connection, as is schematically shownin FIG. 2. By evaluating the vibration spectra determined for eachtightening moment with regard to the average amplitude value, which isproportional to the area under the spectrum curve, it can be concludedthat--as a function of the tightening moment--the averagemagnetostrictive vibration amplitude first decreases with increasingtightening moment, then passes a minimum and subsequently increasesagain. It is to be pointed out here that it is frequently advantageouswhen relative amplitude changes are taken into account in theevaluation. The term "relative amplitude changes" is understood to bethe change of e.g. the average amplitude relative to the averageamplitude of a starting condition. The evaluation of relative signalchanges is particularly advantageous when the sensor 23 is for instanceplaced upon the workpiece 11.

When built-in standardized connecting elements are to be tested, thestress-strain curve and the corresponding magnetostrictive vibrationspectrograms of a standardized connecting element serving as a samplemay first be determined. It must be taken into account here that forobtaining the spectrograms, the coil and sensor must be disposedrelative to each other on an accessible part of the connecting elementin the same manner as will be the case in the built-in standardizedconnecting element. The corresponding data is then available in a memoryof the evaluating circuit as reference values for measuring. When therespective built-in standardized connecting element is tested, thesignal output by the sensor is compared with the stored values of thestress-strain curve in order to provide a measuring result for thestressing condition.

It is also possible to determine reference values of a standardizedconnecting element, to use these for creating reference magnitudes inthe stress-strain curve and to transform them into a computing program.This computing program is then used for example by a microprocessor inthe signal processing means in order to relate the signal producedduring testing of the built-in connecting element to the said referencemagnitudes so as to obtain a measuring result for the stressingcondition.

In a further embodiment of the process according to the invention, aD.C. magnetic field, the strength of which is variable on a quasi staticbasis, is superimposed on the alternating magnetic field, the frequencyof which is continuously varied. This results in a premagnetisation ofthe range of the connecting element which is magnetostructively excitedby the alternating magnetic field. Owing to this measure, the magneticdomain structure in this domain is determined both by the mechanicalstresses prevailing and by the direction and strength of the quasistatic D.C. magnetic field. Thus the magnetic field is quasi staticallymodulated such that its variable strength--for example starting fromfield strength 0--assumes values which are cyclically varied between twofield strength values so that hysteresis appearances of the measuringmagnitude can be registered as a function of the variable D.C. fieldstrength with the parameter of the prevailing stress condition. Magnetichysteresis appearances occur in all ferromagnetic materials, inparticular in those materials which are used for connecting elements.

As shown in FIG. 2, the D.C. magnetic field is thereby produced by meansof a D.C. field coil 20 wound on a hollow cylindrical ferromagnetic core24. The D.C. field coil 20 is supplied with variable direct current by abipolar power supply 22 which is controlled by control and evaluatingmeans 25. Due to the fact that one of the open ends of the hollowcylindrical core 24 rests directly on the nut 15, the D.C. magneticfield extending in the core 24 is continued in the nut, thussuperimposing the alternating field produced by the coil 19.

The D.C. magnetic field, however, can also be led into the structuralcomponent under stress via a ferromagnetic coupling element 26, as isshown in FIG. 4 and 5, by disposing the D.C. field coil 20 on thecoupling element 26. The coupling element 26 then has two functions,namely, on the one hand, transferring the measuring signals from theexcitation site to the sensor 23, and, on the other, guiding the D.C.field flux into the connecting element to be tested. It is also possibleto superimpose a variable D.C. field portion onto the A.C. field coil 19so that only one coil is required for the D.C. and A.C. magnetic fields.

FIG. 10 shows the measuring results obtained in testing severalcorresponding screwed connections under individually different butdefined prestressing conditions. The test is conducted as described inFIG. 9, using a varying D.C. magnetic field which has an additionalaxial effect on the nut.

It goes without saying that the maximum magnetizing field strength needat the most reach the value of the saturating field strength of thematerial to be tested. In comparative tests, however, it must be ensuredthat the field strength is modulated in the same manner every time. Anevaluation of the dependence of the variable D.C. field strength and thevibration spectra detected as a function of the stressing conditionprevailing in the nut shows that a magnetostrictive hysteresis behaviourof the amplitudes is obtained which is dependent on the stressingcondition. The ordinate values of the diagram in FIG. 10 are herebyformed by values which are proportional to the area including themagnetostrictive hysteresis loop. Depending on the prestressing of thescrew or bolt, a parabolic curve of the hysteresis values is obtainedhaving its minimum at stresses below the technical yield point R_(p0),1.This means that stresses above or below the hysteresis minimum have moremarked magnetostrictive hysteresis appearances. In the minimum, theamplitudes of the vibration spectra obtained during cyclic modulation ofthe D.C. field only show minor variations, which results in a smallhysteresis area.

The screw prestressing range in FIG. 10 relative to the technical yieldpoint R.sub.,1 corresponds to the respective stressing area of thediagram in FIG. 1 which diagram shows the dependence of the maximummagnetic permeability on the stress. Since the screwed connection alsohas magnetostrictively positive properties, the hysteresis minimum foundin FIG. 10 can be related to the permeability maximum of FIG. 1.

The latter also occurs below the technical yield point R_(p0),1 and is,as described in Bozorth, at the elastic yield point.

In contrast to all known methods, the test process according to theinvention thus for the first time also provides a simple, easilyfeasible way of very accurately determining the elastic yield point. Itis pointed out in particular that this material parameter is determinedon the basis of signals which are actively provided by the materialitself and are caused by submicroscopical processes in the interior ofthe material. Length and force measurements with their potentialinaccuracies as used in conventional methods are not required here.

It is thus possible by means of the measuring process according to theinvention to accurately and definedly adjust the prestress, oriented atthe elastic yield point. In particular, it is not necessary to know theaccurate numerical value of the mechanical material parametersbeforehand, since the yield point becomes perceptible in the inventivetest process in the magnetostrictive hysteresis values becoming minimalduring tightening of the screwed connection. Consequently, substantiallyincreased safety of assembled screwed connections, without majorfriction losses, is obtained.

When built-in connecting elements are tested, it is likewise possible toclearly identify the prevailing stress condition. This can be carriedout by first determining the stress-strain curve of a standardizedstructural component serving as a sample and then the relatedmagnetostrictive hysteresis behaviour which is functionally dependent onthe stress. The corresponding data is then available in a memory of theevaluating means as reference values for measuring. For determiningwhether the prestress is above or below the elastic yield point, eitherthe appearance form of the hysteresis loop or the magnetostrictivevibration spectrum can be considered, both of which are of a clearlydifferent form below the elastic yield point than above this point. Asan example thereof, reference is made to FIG. 8, the partial pictures a,b of which illustrate vibration spectra below and the partial picturesc,d illustrate vibration spectra above the yield point.

In a further embodiment of the process according to the invention, theaforementioned hysteresis behaviour is determined without a superimposedD.C. field, in that the amplitude of the alternating magnetic field, thefrequency of which is continuously varied, is cyclically varied step bystep between a low and a high amplitude value in such a manner thatafter termination of the cycle, the amplitude of the alternating fieldis at its original value again. Between the amplitude increasing steps,the alternating frequency of the magnetic field is adjusted. It goeswithout saying that the maximum alternating field amplitude need onlyassume such values as are required for magnetic saturation of theconnecting element to be tested.

This approach has the advantage that only one coil is needed fordetermining the hysteresis behaviour and that no D.C. power supply isrequired. The process for testing connecting elements is as describedfor testing by means of a superimposed D.C. field.

It is additionally pointed out that the inventive process for thenon-destrictive determination of the prestressing condition offerromagnetic connecting elements is also suited for materials havingmagnetostrictively negative properties.

I claim:
 1. Process for non-destructive determination of a stressing condition of a selected ferromagnetic connecting element wherein said process comprises the steps of:applying an alternating magnetic field to the connecting element, detecting any magnetostrictive or acoustic vibrations resulting from the application of the alternating magnetic field on the connecting element, obtaining a signal representative of the resulting magnetostrictive or acoustic vibrations detected from application of the alternating magnetic field on the connecting element, and evaluating the obtained signal resulting from the magnetostrictive or acoustic vibrations to determine the stressing condition of the ferromagnetic connecting element.
 2. Process according to claim 1, further comprising the steps of:superimposing a variable D.C. magnetic field on the alternating magnetic field.
 3. Process according to claim 1, further comprising the step of:modulating the D.C. magnetic field in a quasi static manner such that its variable strength, starting from an initial value, is varied cyclically between two field strength values.
 4. Process according to claim 3, including the steps of:varying the frequency of the alternating magnetic field; and detecting a magneto-mechanical acoustic vibration spectrum as a function of the frequency.
 5. Process according to claim 4, wherein the step of obtaining the vibration spectrum during testing the connecting element includes comparing with a vibration spectrum assigned to a stress-stain curve in order to obtain a measuring result for the stressing condition.
 6. Process according to claim 4, including the steps of:determining reference values from a standardized connecting element, using the reference values for creating reference values in the stress-strain curve, and producing the vibration spectrum during the test of the connecting element being related to these reference magnitudes in order to obtain a measuring result for the stressing condition.
 7. Process according to claim 6, comprising the step of: determining a numerical value which is in a functional relation to the area under the determined vibration spectrum and which is assigned a point on the stress-strain curve.
 8. Process according to claim 7, comprising the step of: determining a numerical value which is in functional relation with the area of the magnetostrictive hysteresis curve and which is assigned a point on the stress-strain curve.
 9. Process according to claim 8, comprising the step of: feeding an output signal obtained after evaluation into display means for displaying the measured value or is used for controlling screw means.
 10. Process according to claim 9, comprising the steps of: measuring the frequency shift of individual lines of a spectrum in comparison with a reference spectrum and obtained as a measure for the strain of a connecting element.
 11. Process according to claim 10, comprising the step of: measuring the amplitude change of individual lines of the spectrum in comparison with a reference spectrum and obtained as a measure for the stress in the connecting element.
 12. Process according to claim 11, comprising the step of: obtaining the stress-strain curve by connecting the measured values of the frequency shift and the amplitude change.
 13. Device for the non-destructive determination of a stressing condition of ferromagnetic connecting elements, said device comprising,an A.C. generator (21) of variable frequency, a coil (19) connected with the A.C. generator (21) for generating a magnetic field in at least part of the connecting element (13), an acoustic-electric sensor (23) abutable to the connecting element (13) for detecting any magnetostrictive or acoustic vibrations resulting from the generated magnetic field in at least part of the connecting element, and signal processing and control means (25) for obtaining a signal representative of the magnetostrictive or acoustic vibrations detected from generating the magnetic field in at least part of the connecting element, and for evaluating the obtained signal resulting from the magnetostrictive or acoustic vibrations to determine the stressing condition of the ferromagnetic connecting elements.
 14. Device according to claim 13, characterized by a D.C. generator (22) connected with the coil (19) for generating a D.C. magnetic field.
 15. Device according to claim 14, characterized by a D.C. field coil (20) for generating a magnetic field in at least part of the connecting element (13), said D.C. field coil (20) being connected with a D.C. generator (22).
 16. Device according to claim 15, characterized in that display means (27) for displaying the stressing condition in the connecting element (13) is connected with the evaluating circuit (25).
 17. Device according to claim 16, characterized in that screw means (28) is connected with the evaluating circuit (25).
 18. Device according to claim 17, characterized in that the coil (19 or 20) and the sensor (23) are arranged practically concentrically to each other.
 19. Device according to claims 18, characterized in that the sensor (23) is mounted on a coupling element (26).
 20. Device according to claim 19, characterized in that the coil (19 or 20) is disposed within the coupling element (26).
 21. Device according to claim 20, characterized in that it is provided in the form of a rod-like hand device having a head (33) in which the coils (19 or 20) and the sensor (23) are accommodated.
 22. Device according to claim 21, characterized in that the sensor (23) can be displaced axially in the head (33) against the force of a spring (37).
 23. Device according to claim 22, characterized in that the alternating field coil (19) and the D.C. field coil (20) are essentially arranged concentrically to each other.
 24. Device according to claims 12, characterized in that the D.C. field coil (20) is disposed essentially perpendicularly to the alternating field coil (19).
 25. Device according to claim 24, characterized in that the D.C. field coil (20) is disposed on a coupling element (26), which coupling element (26) on the one hand transfers the measuring signals from the excitation site to the sensor (23) and on the other feeds the D.C. field flux into the connecting element (13) to be tested.
 26. Device according to claim 25, characterized in that the coils (19, 20) are integrated in an automatic hand device. 